1. Technical Field of the Invention
The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as rotary cone drill bits.
2. Description of Related Art
Reference is made to FIGS. 1 and 2, wherein FIG. 1 illustrates an isometric view of a prior art rotary cone drill bit 10, and FIG. 2 illustrates a cross-sectional view of a portion of the prior art rotary cone drill bit 10 of FIG. 1. A leg 12 depends from a body portion 14 of the drill bit 10. The leg 12 includes a bearing shaft 16 which extends in a downward and radial inward direction. The bearing shaft 16 includes a cylindrical bearing surface 18. A cutter cone 20 is mounted to the bearing shaft 16 and supported for rotation by the bearing surface 18. In an alternative implementation, the cutter cone 20 is supported for rotation on the bearing shaft 16 by a set of roller bearings. The shape and configuration of the cone 20, as well its rotatable attachment to the bearing shaft 16, is well known in the art. In sealed bearing implementations, the bearing (journal or roller) between the cone 20 and bearing shaft 16 is lubricated by a lubricant (such as a grease) that fills regions adjacent to the bearing as well as other passages 21 in the rotary cone drill bit in a manner well known by those skilled in the art. This lubricant is retained within the rotary cone drill bit through the use of, for example, a resilient seal in the form of an o-ring 22 positioned in a seal gland 24 between the inner cylindrical surface 26 near the base of the cone 20 and the outer cylindrical surface 28 near the base of the bearing shaft 16. The lubrication system further includes a pressure compensation assembly 27 installed within an opening 29 formed in an upper shoulder surface 25 of the leg 12. The opening 29 is coupled to the passage 21. FIGS. 1 and 2 illustrate an implementation wherein the opening 29 is formed exclusively in the shoulder surface 25. It will be understood, however, that the opening 29 may be formed partially in the shoulder surface 25 and partially in the outer (gage or shirttail) surface 30 of the leg 12 (below shoulder surface 25). Still further, the opening 29 may be formed in the outer surface 30 of the leg.
The outer surface 30 of the leg 12 terminates at a semicircular edge 32 proximal to the cone 20. The region of the leg 12 associated with the surface 30 is known in the art as the “shirttail region,” and the edge 32 is known in the art as the “shirttail edge.” The shirttail edge 32 is provided where the terminal portion of the outer gage or shirttail surface 30 transitions to an inside radial surface 34 oriented parallel to the base of the cone 20 (and perpendicular to the bearing shaft 16) and positioned at the base of the bearing shaft 16.
The outer surface 30 of the leg 12 (below shoulder surface 25) in the shirttail region laterally terminates at a leading shirttail edge 50 and a trailing shirttail edge 52. The leading shirttail edge 50 is especially susceptible to wear during operation of the rotary cone drill bit 10. The prior art teaches two methods for delaying wear of the leading shirttail edge 50. FIG. 3 illustrates a first method in which a layer of welded hardfacing material 40 is applied to the surface 30 extending along at least a portion of the leading shirttail edge 50. The hardfacing material is typically a deposit of tungsten carbide hardmetal 40. The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material 40 wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material. FIG. 4 illustrates a second method in which tungsten carbide inserts 42 are press-fit into holes 44 formed in the surface 30 near the leading shirttail edge 50. While these inserts 42 provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts 42 do not provide protection for the leading shirttail edge 50. The reason for this is that the holes 44 must be located at some appreciable distance from the leading shirttail edge 50 in order for the press-fit to function properly and peripherally retain the inserts 42. For example, a separation d2 of at least 0.125 inches is typically provided from the edge of the hole 44 to the leading edge 50. Thus, the method of FIG. 4 functions to primarily protect the shirttail region near to, but not exactly at, the leading shirttail edge 50. Furthermore, in order to be suitably retained, the press-fit inserts 42 must typically have a thickness t (with a corresponding depth of the hole 44) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5).
Although not explicitly shown in FIGS. 3 and 4, the protection mechanisms shown could alternatively, or additionally, be provided on the leading side surface 54 of the leg 12. This leading side surface 54 is adjacent the outer surface 30 of the leg 12 at the leading shirttail edge 50.
With reference once again to FIGS. 1 and 2, the shoulder surface 25 of the leg 12 laterally terminates at a leading shoulder edge 51 and a trailing shoulder edge 53. The leading shoulder edge 51 is also susceptible to wear during operation of the rotary cone drill bit 10, more specifically when the bit 10 is being removed from the drill hole. This is because the shoulder edge 51 may have to function in a cutting-like or abrasive manner to remove formation materials in instances where the drilled hole has narrowed. The leading shoulder edge 51 and shoulder surface 25 are further susceptible to damage from formation material falling in the drill hole and having to be removed. Wear of the leading shoulder edge 51 and shoulder surface 25 can have adverse effects on the opening 29 and the pressure compensation assembly 27 installed within that opening 29 that perhaps lead to premature failure of the lubrication system.